In recent years, as display panels used in display devices such as digital televisions, organic electroluminescence (EL) display panels that use electroluminescence of organic materials are being implemented in which a plurality of organic EL elements are arrayed on a substrate.
As an organic EL display panel configuration, typically thin film transistor (TFT) drive circuitry is disposed on a substrate, an insulating layer is disposed on the TFT drive circuitry, and a plurality of organic EL elements are arrayed thereon. An organic EL element has, at minimum, a light-emitting layer sandwiched between a pair of electrodes (anode and cathode). Typically, an organic EL element has a hole injection layer and a hole transport layer between the anode and the light-emitting layer, and an electron injection layer and an electron transport layer between the cathode and the light-emitting layer. The hole injection layer, the hole transport layer, the electron injection layer, and the electron transport layer fulfil their respective specific functions of charge injection, charge transport, etc. In the present disclosure, such layers are referred to collectively as “functional layers”. Such an organic EL element is a current-driven light-emitting element. When driven, a voltage is applied between the anode and the cathode, and light is emitted when holes and electrons injected into the light-emitting layer recombine.
In an organic EL display panel, such organic EL elements form sub-pixels in red, green, and blue colors, combinations of adjacent red, green, and blue sub-pixels forming pixels. In such an organic EL display panel, in order to reduce energy consumption and increase longevity, improving light emission efficiency and longevity of the organic EL elements for each of red, green, and blue colors are important. Among red, green, and blue colors of organic EL elements, lifespan of blue organic EL elements tends to be shortest, and therefore increasing longevity of blue organic EL elements is a technical problem for increasing longevity of display devices.
Patent Literature 1, 2, and 3 disclose implementing increased organic EL element longevity by adoption of an organic EL element structure in which a plurality of light-emitting units that include a light-emitting layer (a light-emitting unit being equivalent to a conventional organic light-emitting element without anode and cathode elements) are stacked (this may be referred to as a multiphoton structure, tandem structure, or stack structure, but this disclosure refers to a tandem structure). A tandem structure is a structure in which light-emitting units are stacked on both sides of a charge generating layer (a portion thereof in contact with a light-emitting unit may be referred to as a charge generating layer, an intermediate layer, an intermediate conductive layer, a connector, etc., but this disclosure refers to a charge generating layer). This charge generating layer supplies charge to the light-emitting units above and below it, and therefore light can be emitted from a plurality of light-emitting units in a tandem structure. As a result, light emitted from an organic EL element that has a tandem structure is a sum of light emitted from a plurality of light-emitting units, and therefore current required to obtain a desired luminance is decreased, and longevity of the current-driven type of organic EL element can be increased.
In order to adapt a tandem structure to an organic EL display panel, which light emission colors of light-emitting units to layer and how to layer them in red, green, and blue sub-pixels become important.
Resistance heating, coating application, printing, etc., are methods of film formation for light-emitting layers and functional layers made from organic light-emitting material. When vacuum deposition is used for film formation of light-emitting layers or functional layers of different sub-pixels, a precision shadow mask is typically used that is only open at sub-pixel regions. In such a case, material that forms light-emitting layers or functional layers attaches to portions of the shadow mask other than the openings, and this loss of material is a cause of increased manufacturing cost. Further, accurate alignment of openings of the precision shadow mask to sub-pixels is a factor lowering production efficiency, and in particular when producing large organic EL display panels the alignment of shadow mask to sub-pixels is very difficult.
Thus, from the perspectives of manufacturing costs and enlargement of an organic EL display panel, research is being actively pursued into film formation in which a low-molecular or high-molecular organic material is dissolved in solution or dispersed, and a light-emitting layer and functional layer including organic light-emitting material are formed by a wet process. However, there is a problem in that an organic EL element formed by using a wet process has a shorter lifespan that an organic EL element formed by using vacuum deposition.
Patent Literature 4 discloses a tandem structure in which blue light-emitting units are layered only in blue sub-pixels, in an organic EL display panel, by using a combination of wet and vacuum deposition processes. As a result, longevity and productivity of blue organic EL elements are improved, and manufacturing cost is decreased. More specifically, after forming a first red light-emitting unit including a red light-emitting layer, a first green light-emitting unit including a green light-emitting layer, and a first blue light-emitting unit including a first blue light-emitting layer by using wet processes, a charge generating layer is formed only in a blue sub-pixel region by using a vacuum deposition process. Subsequently, a second blue light-emitting unit including a second blue light-emitting layer is formed by using vacuum deposition so as to cover the first red light-emitting unit in a red sub-pixel, the first green light-emitting unit in a green sub-pixel, and the charge generating layer in the blue sub-pixel. Thus, there is no charge generating layer between the first red light-emitting unit and the second blue light-emitting unit, and the second blue light-emitting unit only has an electron transport function, and therefore light is not emitted from the second blue light-emitting unit in the red sub-pixel. The same is true for the green sub-pixel. According to this configuration, Patent Literature 4 discloses that longevity can be increased, manufacturing cost decreased, and productivity improved for a blue organic EL element.